An organic light emitting element is an electronic element including an anode, a cathode, and a thin film containing a luminous organic compound, formed between the anode and the cathode. A voltage is applied between the anode and the cathode to inject a hole and an electron, whereby the organic light emitting element is driven. Specifically, the hole and electron applied from the respective electrodes recombine in the element (the thin film containing the luminous organic compound), and when the luminous organic compound in an excited state (exciton) produced by the recombination returns to its ground state, light is radiated. The organic light emitting element is an element that utilizes the light to be radiated.
From the viewpoint of a light emission principle, light emissions of the organic light emitting element can be classified into two kinds, i.e., fluorescent emission utilizing a singlet exciton and phosphorescent emission utilizing a triplet exciton. In this case, in principle, a ratio between the probability that the singlet exciton is produced and the probability that the triplet exciton is produced is 1:3, and hence it is found that the triplet exciton is more likely to be produced. Accordingly, in the case of an organic light emitting element utilizing fluorescence, the ratio at which the fluorescence can be utilized as light emission (internal quantum efficiency) is 25% at maximum. Meanwhile, in the case of an organic light emitting element utilizing phosphorescence, its internal quantum efficiency is 75% at maximum.
When an organic light emitting element is applied to a constituent member for a full-color display in a cellular phone, television, or the like, improvements in efficiency and durability of the organic light emitting element have been realized in each of a red color and a green color by utilizing phosphorescence. In a blue color, however, the fact that the durability of an organic light emitting element that emits blue light is problematic and hence its continuous driving lifetime is short has been perceived as a problem. In view of the foregoing, an organic light emitting element that has relatively good durability and utilizes fluorescence has been used in the blue color. However, the element has caused a problem from the viewpoint of luminous efficiency because the probability that a singlet exciton is produced is low as described above.
In this connection, methods proposed in PTL 1 and NPL 1 are each given as a method of improving the luminous efficiency of an organic light emitting element that utilizes fluorescence and whose luminescent color is a blue color.
By the way, a method involving utilizing a triplet exciton, which has not been effectively utilized heretofore, in fluorescent emission has been proposed as a method of improving the luminous efficiency of an organic light emitting element utilizing fluorescence. The method is a method called triplet-triplet annihilation (TTA) and a specific light emission mechanism is as described below. First, multiple triplet excitons are produced in an emission layer and then two of the triplet excitons collide with each other to annihilate. At that time, a singlet exciton is produced. Thus, singlet excitons needed for the fluorescent emission increase in number and hence the fluorescent emission increases in extent. As a result, the internal quantum efficiency of the element becomes 62.5% at maximum and the luminous efficiency improves. Here, when the ratio at which the fluorescent emission can be extracted as light emission to the outside of the element (external quantum efficiency (EQE)=internal quantum efficiency×light extraction efficiency) is calculated, the calculated value exceeds 5% as the upper limit of the related art (when it is assumed that the light extraction efficiency is 20%) and can be set to 12.5% at maximum.
An organic light emitting element using an anthracene derivative as a host for its emission layer has been proposed as an organic light emitting element that emits blue light and utilizes the TTA, and it has been reported that the element has high luminous efficiency. For example, in NPL 1, the external quantum efficiency of the organic light emitting element that emits blue light is about 7%, which exceeds the upper limit of the related art. However, even when the TTA is utilized, an upper limit for the external quantum efficiency of the organic light emitting element is 12.5% and hence the efficiency can be said to have room for additional improvement. By the way, in NPL 1, an aromatic amine derivative (α-NPD) is incorporated into a hole transport layer and the aromatic amine derivative may cause a reduction in luminous efficiency.
The aromatic amine derivative generally has strong electron-donating property. Accordingly, the derivative interacts with a constituent material for an emission layer and the interaction is responsible for emission quenching. In view of the foregoing, there has been proposed an organic light emitting element using an organic compound except the aromatic amine derivative as a constituent material for its hole transport layer for an improvement in luminous efficiency. For example, PTL 1 discloses, as organic compounds except the aromatic amine derivative, an anthracene derivative, arylethylene and arylacetylene derivatives, and a polyphenylene hydrocarbon. In addition, in Examples of PTL 1, an organic light emitting element using an anthracene derivative as its hole transport layer shows an improvement in luminous efficiency in each of the red color and the green color.
However, the triplet energy level (T1) of the anthracene derivative as a constituent material for the hole transport layer is of about the same magnitude as that of a host for the emission layer of the element. Accordingly, when the anthracene derivative is used as a constituent material for the hole transport layer in the organic light emitting element utilizing the TTA proposed in NPL 1, no triplet exciton can be trapped in its emission layer. Accordingly, a triplet exciton leaks to a layer adjacent to the emission layer (such as the hole transport layer), which leads to a reduction in luminous efficiency. Therefore, an organic compound whose T1 is higher than that of the anthracene derivative (1.8 eV or less) has been desired.
However, PTL 1 discloses that an organic light emitting element using an organic compound except the anthracene derivative, the compound being considered to have a high T1, is not improved in luminous efficiency to a very large extent in each of the red color and the green color, and moreover, the element causes an increase in driving voltage. The same may hold true for an organic light emitting element that emits blue light.